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VM99S
4-4-6 Synchronization
1. Normal Mode
The sync pulse from FBT (31 kHz~95 kHz) via R812, R831, C815, R826, D824, C814 and R816 to pin 4
of I801 to keep I801 synchronized with horizontal sync input frequency.
2. Power Saving Modes: Off mode
Because there is no pulse from FBT, so the free-run frequency is decided by R817 and C814 and the
SMPS works at 22 kHz.
3. Override
The horizontal free run frequency is about 62.5 kHz under override condition, SMPS is synchronized to
this frequency.
4-4-7 O.V.P.
If the auxiliary voltage is higher than zener voltage ZD807 (20 volts) and makes pin 3 of I801 higher than
1 V, pin 6 duty cycle is limited to have the OVP activated.
4-4-8 O.P.P.
The excess current of T802 through R813, R814, R834, C813, R865 and R864 can develop enough voltage
on pin 3 then limit the power delivered because the pin 6 duty cycle is limited too.
4-4-9 High Voltage Generation with F.B.T.
1. The H. V generation circuit combines T101 F.B.T. with I101 TL494/KA7500 PWM control circuit.
2. When I101, Vcc(pin12) reaches at 12V, pin 5,6 gets a freerun sawtooth waveform about 25KHz, and
approximately 5.7Vp-p, when voltahe at pin 15 is higher than pin 16 (5V reference) then release pin 3 to
be controlled by pin 1 feedback signal and compare with pin 5 to output PWM.
3. HFLB1 is used to trigger pin 5 via Q112, to synchronize the deflection circuit.
4. PWM output will drive Q116 & T101 to gererate high voltage (26KV), T101 pin 11, 15 becomes DC/AC
feedback, with the voltage link to I101 non-inverting input pin 1 to stabilize H.V (26KV) and I101 can be
adjusted to control H.V value.
5. When X.R.P occur, the hunlock will keep high to pull down I101 pin 2 and shut down PWM output.
4-5 Video Amplifier Section
1. RGB signal inputs are terminated by R501, R531 and R561 then pass through the coupling capacitors
C501, C531 and C561 to IC501 LM1267 preamplifier.
2. The amplifier RGB signals (0~4 Vpp) are adjusted by I
2
C bus from I501, pin 23 is for clamp pulse which
comes from pin 16 of TDA4841.
3. The video output stages are amplified by I901(LM2465). The RGB cathodes cut off are adjusted by I
2
C
bus control by internal icon.
4. Under override condition, “CHECK SIGNAL” will show up on the screen.
4-6 OSD (On Screen Display) Circuit
